Chromatography, in modern separation science addresses the separation of components found in a mixture on the basis of their differing behavior between a moving phase and a stationary phase, which phases are in continuous and direct contact. When the moving phase is gaseous it is known as gas chromatography. Interaction with the stationary phase is of two forms; as surface adsorption, or solubility in a static liquid phase. One, (former) is called adsorption chromatography, and the other (latter) is called partition chromatography because each analyte partitions itself between the moving phase and the stationary phase by reason of its chemical nature.
In gas chromatography, the instrument utilized is a gas chromatograph which includes three general components: an inlet system; a column containing the stationary phase, and a detector. The inlet system may accommodate liquid or gaseous samples. The liquid samples are immediately volatilized in the inlet. The sample may be subsequently applied directly into the column, may be split so that only a certain portion goes to the column, or, in the case of gaseous samples, may be trapped in a stationary state and later expelled from the trap by a purging mechanism and directed into the column, again, either in total or in part.
In gas chromatography over the last 20 years the component showing the most advancement is the column, originally characterized as packed columns. The packed columns were relatively large open tubes packed with the stationary-phase-coated particles. Typically, using a carrier gas such as helium for the moving phase, a mixture containing various components was introduced onto the column where the components were separated in time so that the duration of a detected peak for each component ranged from several seconds to a few minutes. The most common detectors in original use were thermal conductivity or hydrogen flame-ionization. Both detectors were non-specific in their response and both required complete temporal separation of each of the individual components of the sample for proper analysis.
To provide additional information from each component, a two-dimensional detection system, such as a mass spectrometer was attached to the gas chromatograph. The result of this modification was the creation of a gas chromatograph/mass spectrometry GC/MS hybrid instrument. The GC/MS instrument was the first instrument requiring a computerized data system (GC/MS/DS) which became the dominant analytical instrument in modern laboratories. The typical mass spectrometer could obtain a complete mass spectrum, or scan, in about one second so as to enable numerous scans within the time frame of a single eluting peak. There followed the development of fused silica capillary columns and bonded stationary phases. Consequently, the peak widths of the gas chromatography (GC) were reduced to a few seconds, or less, creating substantial demands upon the speed and performance of the mass spectrometer.
Manufacturers of mass spectrometers attempted to accommodate the new demand for increased speed in scanning but failed to reach a performance in which no chromatographic information was lost while simultaneously maintaining adequate sensitivity. Various improvements such as magnetic sector instruments utilizing laminated magnets, smaller magnets and variant geometries attained scan speeds approaching three full mass range scans per second. One manufacturer, using a magnetic field via electromagnetic coils only, reached scan speeds up to 50 full scans per second. Such an increase in rate of scanning diminished the sensitivity of the instrument; and effective scanning with sufficient sensitivity for gas chromatographic analysis is limited to a maximum of about 10 scans per second. Quadrupole instruments designed with decreased length of the quadrupole filter and increased extraction potential attained rates of two to three s per second with reasonable sensitivity.
Time-of-flight mass spectrometry (TOFMS) has enjoyed the potential of producing mass spectra at a rate of 5,000 to 10,000 scans per second. However, used in its original embodiment, TOFMS, when combined with a data system, employs a technique known as time sliced detection (TSD) which limits this rate approximately to one full range scan per second to maintain reasonable sensitivity. In TOFMS, the ions are extracted from the ion source and are accelerated to a constant energy and are allowed to separate on the basis of the velocity (hence mass). An exact measurement in the time-of-flight over a fixed distance provides information for subsequent mass assignment. In TSD, only a small fraction of the mass range is actually measured after each extraction and this is accomplished by collecting data from a small time period, usually in the range of 2 to 20 nanoseconds wide, from each extraction. Varying the time delay between extraction and data collection for successive extraction cycles provides the information for a complete mass axis scan.
In the art described above, ions are measured as a function of their mass in a time dependent sequential manner. Since only the ions of a single mass are being measured at any given time, individual ion statistics for every mass are lost whenever other ions are being measured. Mass spectrometers that operate this way are called scanning mass spectrometers. Another means of ion measurement involves array detection. In array detection, ions throughout the mass range are measured simultaneously or sequentially from an event simultaneous to all ions. Spatial array detectors are comprised of multiple miniature ion detectors across whose dimenstions the ions are dispersed as a function of their mass. By this means, all the ions present are measured simultaneously. Readout mechanisms for this technology are cumbersome and time consuming, and to date, no applications to chromatography have been documented.
Temporal array detectors measure in the time domain either in a synchronous or a nonsynchronous manner. Synchronous detectors measure in the frequency domain while nonsynchronous detectors measure time. Simultaneous frequency detectors confine the ions in electric and magnetic fields and utilize Fourier transform techniques to detect and quantify all of the ions present at the same time. These types of array detectors have been applied to chromatography attaining 2-5 spectra per second with moderate to poor sensitivity. Another type of frequency array detector is the ion trap mass spectrometer. In this device, after an ionization event, all of the ions are trapped in an RF field. Changing the amplitude and/or frequency characteristics of the field allows the ions to be measured sequentially in mass by increasing each iso-mass orbit until a fixed ion detector is encountered. This is an example of an array detector that measures all ions subsequent to an ionization and trapping event. Spectral production rates up to 50 per second have been accomplished with this device; however, at a significant sarcifice of sensitivity and resolution. For chromatographic applications, rates in the 2 to 10 spectra per second range are more typical. The presently described unit utilizes nonsynchronized temporal array detection called time array detection (TAD). When several full mass spectra can be obtained over the time required to elute a single compound, information about the way in which the eluant composition changes with time can be realized. The ability to use these data to detect and distinguish compounds whose elution profiles overlap has been demonstrated by several early practitioners of GC/MS. One of the first publications in which this process was demonstrated and roughly described was that of J. E. Biller and K. Biemann in 1974 (7 Anal Lett 515-528). Other work followed with variations on the methods used for data analysis. R. G. Dromey and M. J. Stefik in 1976 (48 Anal Chem 1368-1375) analyzed the elution peak profile by the determination of m/z values (mass per unit change and time) contained in the spectrum of only one of the coeluting compounds. B. E. Blaisdell and C. C. Sweeley in 1980 (117 Anal Chem Acta 1) applied a curve fitting algorithm to detect and distinguish coelutants. A least squares analysis was employed by F. J. Knoor, H. R. Thorsheim and J. M. Harris in 1981 (53 Anal Chem 821) and factor analysis was used by M. A. Sharaf and B. R. Kowalski in 1982 (54 Anal Chem 1291-1296). Despite these efforts and apparent success with model examples of data sets, and despite general availability of at least one implementation of these algorithms with commercial GC/MS instruments, the technique, sometimes referred to as deconvolution, has not been significantly employed. Its lack of successful application is not due to a lack in the sophistication of the algorithms employed but rather the insufficient quality and density in the data available. Advances in chromatography which have resulted in shorter peak widths and lower eluting quantities further degraded the ability of traditional mass spectrometric detectors to provide data of sufficient quality and density for chromatographic deconvolution. Thus the art of chromatographic deconvolution was conceived before its implementation was practical. It is important to note, however, that the intention of the prior art in chromatographic deconvolution was to resolve components unresolved by normal chromatography through the use of the spectral information. Since this was not practically achievable, there was no effort given to achieve reduced analysis time. The present apparatus and process achieves reduced analysis time by compensating for an intentional reduction in chromatographic (time) resolution by deconvolution processes. Until the achievement of spectral data of sufficiently high quality and density was realized such an approach could not be anticipated. The obtention of such high quality data is a significant indicia of present invention.
Accordingly, the present invention has as its principal object an extension of TOFMS by use of procedure and apparatus for time array detection (TAD) permitting the reduction of time required for analysis by use of time compression chromatography with sensitivity and lost resolution sacrificed by temporal compression completely recovered by high density data acquisition and a deconvolution of overlapping chromatographic peaks.
Another object of the invention is to achieve the method objectives by use of available instrument components such as an integrating transient recorder which provides sufficient data to achieve mathematical deconvolution by processing mass spectral information.
Still other objectives are to extend instrumentation in mass spectral analysis for fast and sensitive usage.
Other objects in economy and simplicity and saved time in analysis will be appreciated as the description proceeds.